54 EXPERIMENTAL PHYSIOLOGY. 



break induced shock as before. Then change the wires to the 

 boots so that the anode is nearer the muscle, and stimulate with 

 the same break induction shock. 



Increase the direct current and test the condition at the 

 kathode and anode as before. A certain strength of current is 

 necessary to produce marked changes in the region of the 

 kathode and anode. It is possible to ^ompletelyj^pTock the 

 passage of impulses by means of a direct current in the anodal 

 region. The reverse effect is obtained in the kathodal region. 



These electrotonic changes in the nerve only last for a short 

 time after which the reverse effect may occur. By reversal it is 

 meant that, e.g., around the kathode the stage of increased 

 excitability finally gives way to a condition of lowered excita- 

 bility. This reversal developes more quickly with a strong 

 current. 



STIMULATION BY CONTINUOUS DIRECT CURRENT. 



A direct electric current will produce stimulation of a nerve 

 or muscle when there is a quick change in intensity. This occurs 

 at the make or break, but apparently not during the passage of 

 the current. That there is an effect during the passage of the 

 current is shown in the following experiment. 



Experiment 24. Cut the skin on the ventral surface of a pithed 

 frog so that the sheet of abdominal muscles is disclosed. The 

 rectus abdominis is divided into right and left halves by a 

 longitudinal band of connective tissue, the linea alba. Each 

 half is further subdivided into bellies by five transverse tendi- 

 nous intersections. Place one electrode at the anterior end 

 of either the right or left abdominis, and the other at the 

 posterior end, having a reversing key in the circuit. It will 

 be found that at the kathode of each belly, i.e., where the 

 current leaves the muscle, there is a slight ridge due to con- 

 traction of the muscle as long as the current is passing. If the 

 direction of the current is reversed the contraction again occurs 

 at the kathode. 



It can be shown that the stimulus caused by making a direct 

 current arises at the kathode, while that caused at the break 

 arises at the anode. If the kathode and anode are placed far apart 



